G protein coupled receptors (GPCRs) represent the largest protein superfamily in humans, with nearly 1000 members. They are seven transmembrane receptors that coordinate intercellular communication via the transduction of a wide range of stimuli involved in sensation, neurotransmission, development, emotion, cognition, and function in the CNS, endocrine and immune systems, Chemokine receptors are an important class of GPCRs that are best known for their pivotal role in immune surveillance, where they control the migration and activation of leukocytes in an effort to detect and resolve physiological abnormalities such as cancer and infection. However, inappropriate expression or regulation of these receptors is associated with an extraordinary number of pathologies including inflammatory diseases, cancer and AIDS; thus there is significant interest in developing small molecule receptor antagonists that block the function of specific chemokine receptors. Until recently, GPCRs had eluded structure determination due to challenges in receptor expression and crystallization. However, new technologies have emerged which has made the viability of determining GPCR structures indisputable. To this end, our primary goal is to obtain structural information on chemokine receptors and receptor complexes that can aid drug discovery efforts aimed at improving affinity, efficacy, and selectivity. Accordingly, in collaboration with the PSI network, we will apply novel technologies for the expression, purification and crystallization of GPCRs from the chemokine receptor family, with the goal of determining at least two different receptor structures and multiple co-complexes by the five-year endpoint. To maximize the capabilities of the PSI centers in generating purified protein, and to acquire insights into the dynamic aspects of receptor function, the crystallographic work will be complemented with biophysical studies. Radiolytic footprinting will be developed and applied to map the binding interfaces between chemokines and receptors and to determine information on activation mechanisms. The interaction of pathogenic proteins with chemokine receptors will also be investigated, specifically, the CCR5 receptor with the HIV glycoprotein gp120, and the DARC receptor with the malarial docking protein, DBP. Site Directed Spin Labeling with Electron Paramagnetic Resonance (SDSL-EPR) will be used to characterize the conformational changes associated with ligand binding. All of these studies will be augmented with computational modeling methods in order to rationally guide the experimental construct design and to interpret the biophysical data in a 3D context